VDOT adjusts signals for safety

Robert Thomson
Columnist June 4, 2011

Dear Dr. Gridlock:

I really like a feature that has begun showing up here in the turn lane after the leading green arrow and green circle: a short-timed, following green arrow, which helps to clear the turn lane.

Robert Thomson is The Washington Post’s “Dr. Gridlock.” He answers travelers’ questions, listens to their complaints and shares their pain on the roads, trains and buses in the Washington region. View Archive

I note that the green circle/green arrow lights have a very clear explanation posted between them at all left-turn lane intersections regarding “yielding on the green circle.”

— Sharon Egan, Reston

Writing to me during our continuing discussion of left-turn strategies, Egan said she encountered the “following green arrow” heading west on Sunrise Valley Drive at the intersection with Soapstone Drive in Reston. She said the left-turn lane to Soapstone has the following green.

But in pointing out what she took to be a convenience for drivers, Egan helped the Virginia Department of Transportation identify a potential problem at a handful of intersections and launch a fix.

That problem isn’t created simply by having following green arrows. As Egan points out, they often stop congestion. Multiple sequences of green arrows can improve the efficiency of intersections by clearing left-turn lanes where the turning traffic is heavy or there’s a very short turn lane. But in that situation, the green turn arrow should be the only opportunity for drivers to make the left. No green arrow, no turn.

An unusual configuration at Sunrise Valley Drive, on the other hand, created an opportunity for what traffic engineers call a “yellow trap.” A sequence of signals that includes a solid green circle between leading and following green arrows can be the start of a yellow trap.

Think of typical driver behavior, said Randy Dittberner, VDOT traffic engineer for Northern Virginia. The left-turning driver moves out into the intersection on the solid green circle and waits for a gap. Then the solid green circle turns yellow and the driver starts to get anxious, knowing that the turn needs to be made soon.

The driver assumes that the oncoming drivers see the same yellow signal and will slow down. But if for some reason the signals don’t match, the turning driver may be facing a bigger challenge than the driver realizes.

Dittberner said VDOT’s goal is to avoid any light sequence in which turning drivers might face a yellow trap. A few years ago, he said, VDOT adjusted the settings on signals at 800 intersections with this in mind.

But the sweep missed the configuration at Sunrise Valley Drive. So after VDOT saw Egan’s letter, it set out to adjust the signal she described as well as those at 18 other intersections where the same situation could have occurred. (VDOT has signals at more than 1,400 intersections in its Northern Virginia territory.)

Challenging turns

I think I just reinforced the case I’ve been making that one of the most challenging moves we drivers make on a daily basis is an ordinary left turn.

In our discussions of driver responsibilities, I’ve said that it’s up to the lead driver at the intersection to decide when it’s safe to proceed. Joe Anderson of Arlington wrote that he was disappointed in a letter about left and right turns from Ann Wass and my response to it [Dr. Gridlock, May 1]. “Wass questions the patience of people who, sitting in line behind her, want her to make a left turn, when safe, without the benefit of an arrow,” Anderson wrote [Dr. Gridlock, June 2]. She prefers to wait until the next cycle and the next arrow.” I said Wass was the only one in a position to decide when she could make a safe turn.

Dear Dr. Gridlock:

Thank you for publishing Joe Anderson’s letter about how to properly make a left turn without an arrow, by easing into the intersection when the light is green and using the yellow light if there is no opportunity to turn before then.

I would only add that so long as you are in the intersection when the light turns red, you have the right-of-way to clear the intersection over vehicles that just got the green light. This is what I was taught in traffic school in Los Angeles, where no one would ever get to turn left if people didn’t make use of red lights.

— Paul Wolfe, Lanham

Dear Dr. Gridlock:

Thanks for sticking up for me. Actually, most of the intersections where I regularly turn left are regulated by arrows, so we all have to wait anyway. And I still maintain that one may make a right turn on red, after a full stop, if it is safe to do so.

However, at my doctor’s suggestion, I’m now walking to work three days a week, so all this turning stuff is moot. Now I only have to be concerned about drivers who don’t yield to pedestrians in crosswalks.

— Ann Wass, Riverdale

Dr. Gridlock also appears Thursday in Local Living. Comments and questions are welcome and may be used in a column, along with the writer’s name and home community. Write Dr. Gridlock at The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or
e-mail drgridlock@washpost.com .

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